Field of the Invention
The present disclosure relates to an organic light emitting display device, and more particularly, to an organic light emitting display device which can reduce operating voltage and improve quantum efficiency and lifetime.
Discussion of the Related Art
Image displays used for displaying a variety of information on the screen are one of the core technologies of the information and communication era. Such image displays have been developed to be thinner, lighter, and more portable, and furthermore to have high performance. With the development of the information society, various demands for display devices are on the rise. To meet these demands, research on panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), field emission displays (FED), organic light emitting diodes (OLED), etc. is actively under way.
Among these types of flat panel displays, the OLED devices are advantageous in that they can be fabricated on a flexible substrate such as plastic, operate at a low voltage of 10 V or less, have lower power consumption, and deliver vivid color reproduction, as compared with plasma display panels or inorganic light emitting displays. Also, the organic light emitting display devices are spotlighted as the next-generation display devices that provide rich colors for its ability to render full color images using three colors—red, green, and blue.
An organic light emitting display device can be formed by sequentially stacking an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode. An exciton is formed by the recombination of electrons and holes injected from the two electrodes. A singlet exciton is involved in fluorescence, and a triplet exciton is involved in phosphorescence. Recently, a shift from fluorescent materials to phosphorescent materials is taking place. This is because the fluorescent materials can only use about 25% singlet excitions formed in a light emitting layer to generate light, with 75% triplet excitons lost as heat, whereas the phosphorescent materials have a luminescence mechanism that converts all the excitons into light.
A luminescence process of a phosphorescence light emitting diode will be explained briefly. Holes injected from the anode and electrons injected from the cathode meet at the host material of the light emitting layer. That is, electron and hole pairs mostly meet at the host because of the high concentration of the host although some of them meet at the dopant. In this instance, the singlet excitons formed at the host undergo an energy transition to a singlet or triplet state of the dopant, and the triplet excitons undergo an energy transition to a triplet state of the dopant.
Because the excitons transferred to the singlet state of the dopant are then transferred to the triplet state of the dopant by intersystem crossing, the first destination for all the excitons is a triplet level of the dopant. These excitons formed are transferred to a ground state and generate light. In this case, if the triplet energy of a hole transport layer or electron transport layer adjacent to the front or back of the light emitting layer is lower than the triplet energy of the dopant, a reverse energy transition takes place from the dopant or host to the hole transport layer or electron transport layer, which results in a significant decrease in efficiency. Accordingly, the triplet energy of the hole/electron transport layers, as well as that of the host material of the light emitting layer, plays a highly important role in phosphorescent light emitting diodes.
In order to stably run an organic light emitting display device, it is important that holes injected from the anode and electrons injected from the cathode form excitons while maintaining charge balance in the light emitting layer. However, an excess of holes are left after the exciton formation because the holes have faster mobility than the electrons, and this excess of holes generate charged polarons in the light emitting layer or electron transport layer, thus making the device unstable due to exciton-polaron quenching, etc. In this regard, there are ongoing studies to improve the efficiency and lifetime of organic light emitting display devices by maintaining charge balance in the light emitting layer.